Mercaptopurine, which has a structure
was developed in the 1950s, and was one of the first antileukemia drugs. Though the anhydrate structure is also known, mercaptopurine is sold as the monohydrate form under the commercial name of Purinethol. Mercaptopurine acts as a purine inhibitor in the body, and has shown high activity (IC50=0.5 μM) in some cancer lines. However, the commercial form is plagued by low water solubility (0.249 mg/mL at 37° C.) which may affect its bioavailability. The anhydrous form is more soluble, but suffers from poor stability. Thus, a need exists for a mercaptopurine form that is both soluble and stable.